1. Field of The Invention
The invention relates generally to an apparatus for measuring the coefficient of friction (COF) of a surface. In particular, the present invention relates to an apparatus for measuring the coefficient of friction of a surface of a railroad rail. The invention provides a tribometer with standard train wheels (cart wheels) that is either pushed in front of a hi-rail vehicle, or pulled by a track geometry car (or rail vehicle), which measures and records data representative of the coefficient of friction of the rail tread surface and the rail gauge surface of railroad rails. The tribometer has testing wheels, and uses "dynamic braking" to slow the testing wheels to the point of creepage, that is, just prior the point of slippage of the testing wheels upon the rails. The tribometer calculates the speed of the tribometer's testing wheels and compares it to the speed of the tribometer's cart wheels to calculate the creepage torque on the testing wheels. The creepage torque is used to determine the coefficient of friction of the rail surfaces.
2. Description of Prior Art
Railroad rails are extensively lubricated by the railroad industry to reduce both the wear of the rails and railroad car wheels, and to reduce locomotive fuel consumption. Early experiments on the benefits of lubrication utilized methods of determining lubrication effectiveness by monitoring changes in both rail/wheel forces and temperatures, as well as by visual inspection. Although useful for site-specific purposes, the measurement of forces and temperatures that changed with lubrication levels had several major drawbacks. These drawbacks included: the use of instrumentation that was usually "hard wired" to a single site and thus could not be easily relocated; the use of instrumentation that both was expensive to monitor, and required several iterations of data reduction; the comparison of raw data between different sites (before adjusting for train influences), which was difficult, if not impossible; and, the need for specific train makeup and input information such as train speed, car weight, car type, wheel profile, and recent braking history, all of which had a major effect on results, requiring careful interpretation of the data, and which made accurately determining lubrication effectiveness extremely difficult.
During the past decade, lubricating the wheel/rail interface by the railroad industry has further increased, providing improved efficiency by reducing wheel/rail resistance while extending the wear life of wheels and rails. As part of a train energy efficiency program between 1983 and 1990, the Association of American Railroads (AAR) investigated a number of methods for more accurately monitoring the effectiveness of lubrication. The AAR conducted a number of test programs at the Transportation Test Center (TTC) in Pueblo, Colo. The AAR and major railroads found that energy savings of 5-15% could be obtained with the proper application of lubricant. Since these studies were started, attempts at detailed measurements were conducted to investigate just what was the "proper application" of lubricant.
These investigative measurements were calculated, for the most part, utilizing "secondary" information, that is, information of several characteristics only tangentially related to the measurement of COF. This "secondary" information gathering required, for example, the action of a train, a moving wheel set, the monitoring of locomotive energy output, and other labor intensive and expensive actions, all to provide data that could be interpreted to determine the effectiveness of lubrication. The need for a "primary" measurement apparatus, one that took measurements directly from the rail surfaces, that was not dependent on such extensive actions, for example a passing train or obtaining information on individual train characteristics, became readily apparent early on in the AAR's energy program. The present invention is this "primary" measurement apparatus.
Over the last forty years, several devices for measuring the coefficient of friction, on both railroad rails and other surfaces, have been presented to the public. For example, U.S. Pat. No. 2,496,405 to Foufounis et al. discloses a device for measuring the degree of adhesion of vehicle wheels to a road, railway track or the like (i.e., the friction coefficient of the latter surface) wherein in one embodiment, a pair of coaxial coasters or friction wheels are made to ride upon the surface on which the vehicle moves, providing a spring which is adapted to urge the wheels toward the ground. A rotary screw and a rotary nut cooperating with the rotary screw are adapted to respond to the frictional drag of the friction wheels.
U.S. Pat. No. 3,033,018 to Haggadone discloses a wheel friction indicator comprising a displaceable metal contact block that is mounted in a gap in one rail of a track. When a railway car passes over the contact block, any frictional wheel drag, which can be caused by a defect in the axle journal bearings, will cause the contact block to rotate in the direction of movement of the car. The extent of angular displacement of the contact block is dependent on the magnitude of the frictional drag.
U.S. Pat. No. 3,992,922 to Noble discloses apparatus to predict the coefficient of friction, wherein the apparatus is placed about both rails of a railway so that both wheels of each axle of a railway vehicle can be inspected or analyzed as the vehicle is pushed towards a hump area. Braking elements extend parallel to the rails, are activated by fluid pressure, and frictionally engage opposite sides of the rotating wheels.
U.S. Pat. No. 4,098,111 to Hardmark et al. discloses a method and an apparatus for measuring road or runway properties, thus providing moving vehicles the data to compute the required retardation on a prevailing substructure. At least one measuring wheel is incorporated in a wheeled vehicle, the measuring wheel brought to engage against the substructure, at least during the measuring cycle, and to move over it with a predetermined slip, the value of which is set in relation to the speed of the wheeled vehicle.
U.S. Pat. No. 4,779,447 to Rath discloses a method for determining the coefficient of friction of a roadway. A tire on a vehicle is pressurized during normal brake operation, wherein one of the vehicle wheels is braked at a higher brake pressure than the other wheel. The method then compares the rotational speed of one wheel with the rotational speed of the other to determine the wheel slip from the difference in the rotational speeds of the wheels.
U.S. Pat. No. 4,811,591 to Antoine discloses a device for checking the surface condition of materials that will be coated with a layer of another material for protection, for either altering the material's appearance, or for subsequent assembly with other materials. The Antoine device comprises two measuring wheels. One of the wheels is progressively braked until that wheel slips along the surface of the testing material. The braking torque is measured at that time as an indication of the relative value of adhesion of the material.
U.S. Pat. No. 4,958,512 to Johnsen discloses a method and device for measuring the coefficient of friction of a surface using a wheel and a surface, particularly pneumatic tires and the surface of runways and roads.
U.S. Pat. No. 5,331,839 to Schmidt discloses a method for determining the coefficient of friction of a road surface, wherein brake pressure on a non-driven wheel or measuring wheel is increased until a tendency to lock occurs.
Many prior art tribometers used by the railroad industry are hand-operated devices. Typically, they are used in the field by railroad inspectors for spot-checking lubrication effectiveness. Since they are hand-operated devices, the speed of testing and the length of rail tested are limited to the walking speed and range of the track inspector. These devices also can weigh up to 45 pounds, making long inspection sessions difficult.
The railroad industry ran into several hurtles, overcome by the present invention, in its attempt to construct an automated tribometer. They include, among others: (1) the problem of the build up of lube, dirt, and grease in and around the device which prevents accurate measurements; (2) the problem of interference from communications equipment, for example, the operation of two-way radios and cellular phones, which can interfere with the operation of an automated tribometer, including the data collection; and (3) the disruption in operation of the apparatus upon the build up of ice, snow, and water on the tribometer, where such varying weather conditions cannot interfere with the proper operation of a dependable tribometer.
Further, preliminary investigations revealed several potential problems with moderately loaded, (100-125 lbs.) braked-wheel systems. In such designs, energy dissipation was a problem. At 35 mph, the peak power dissipation reached as high as approximately 6 HP per measurement surface. Yet, if the brake force was repeatedly ramped up until impending slip is detected, the average power dissipation is approximately 3 HP per surface. The AAR has examined some commercial brake hardware that operates in this range and, unfortunately, has found this type brake assembly too large and heavy for use with an automated tribometer. Similarly, electro-dynamic brakes appear to be inapplicable because of their weight.
Thus it can be seen that there is a need for an automated tribometer that is capable of conducting rail inspection over long, continuous segments of rail, and at higher speeds than allowed in the prior art. The tribometer must be dependable, and protected from the various adverse weather and rail conditions it will encounter. Further, the apparatus must overcome the several disadvantages of the present COF rail testing equipment. It is to the provision of such an apparatus that the present invention is primarily directed.